Add schemas for some astropy.modeling models

As I am reading through this and trying out pyasdf I tried creating schemas for `astropy.modeling` models commonly used for WCS transformations. So here they are.

I think only models which don't define an analytical inverse need the `inverse` definition in the schema so only the polynomials have it here. If this is acceptable I'll add the rest of the polynomials.
I think `[[1, 2], [3, 4]]` is clearer, but maybe that's just me.  I guess I don't have a strong feeling as long as it's clear from documentation/examples.